Production of hollow metal microcylinders from lipids

ABSTRACT

Process for making metallic microcylinders from pre-treated diacetylenic lipid tubules which includes placing the tubules into an electroless plating bath containing a metal plating reagent, depositing by electroless plating on the surfaces of the tubules enough of a metal to make the tubules electrically conducting, separating the tubules from the plating bath, treating the tubules to remove the lipid and form the metal microcylinders, washing and drying the microcylinders to produce the metal microcylinders having aspect radio of about  12,  weight average length of about  20 μ, weight average outside diameter of about  1.5 μ, and weight average wall thickness of about a quarter of one micron.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention pertains to hollow metal microcylinders producedfrom diacetylenic lipids by self-assembly and to a process for makingsame.

[0003] 2. Description of Related Art

[0004] Organic tubules formed from a diacetylenic lipid by self-assemblywere first publicly described in 1984 and achieved by a process oflipsomomal cooling. The lipid, i.e.,1,2-bis(10,12-tricosadiynoyll)-sn-glycero-3-phosphocholin (DC-8,9-PC),was heated in water above 50° C. and cooled. Upon cooling below themelting temperature of 42° C., the liposomes were observed to convert totubules of sub-micron diameter by self-assembly. Later it wasdemonstrated how the tubules could be generated from the same lipid bythe addition of water as a non-solvent to a solution of the lipid inalcohol or propylene glycol. The tubules were allowed to grow forperiods up to 6 months and resulted in average lengths of up to 100microns. Still later, the role of alcohols in tubule formation wasfurther investigated. The lipid, i.e., DC-8,9-PC, in alcohol/watersolutions, was heated to 60° C. and tubules were formed by cooling toroom temperature. The solvent 85% methanol was found to yield tubuleswith the greatest lengths with an average of 65 microns.

[0005] Preparation of metallized derivatives from lipid tubules wasreported in 1987. Permalloy-coated tubules have been used in compositesto produce high-dielectric, low-loss materials by aligning the tubuleswith a magnetic field and similar technique was used to producecomposites with significant ferromagnetic properties. The prior arttubules were not electrically conducting probably due to the fact thatelectroless metal plating was conducted to the point when bubblingcommenced indicating that the bath was not exhausted and an insufficientamount of metal was plated on the tubules. A successful application ofthis technology was demonstrated in 1992 by aligning metallized tubulesin a magnetic field and cast in epoxy. Subsequently, the epoxy wasetched away and the surface sputter-coated with gold. This structure wasused to show a very low vacuum field emission of less than 10μA.

[0006] Progress in the application of tubule technology has been heldback by the non-uniformity and incompleteness of the metal coating.Further, the coating consisted in a large part of metals and metallicoxides, such as nickel and nickel oxides, rather than a metallic alloy,such as nickel alloy.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

[0007] An object of this invention is a process for making hollow metalmicrocylinders from diacetylenic lipids by self-assembly characterizedby using as received technical grade diacetylenic lipids withoutpreliminary purification, resulting in a more efficient use of lipids.

[0008] Another object of this invention is hollow metal microcylindersin free-flowing powder form.

[0009] Another aspect of the invention is the electrically conductivemetal microcylinders.

[0010] Another object of this invention is electrically conducting metalmicrocylinders made from a diacetylenic lipid by self-assembly which aredevoid of the lipid.

[0011] These and other objects of this invention can be achieved bymaking the microcylinders from diacetylenic lipids by self assemblythereof to produce lipid tubules, and then metallizing these tubules byplating at least one metal thereon to make the metallized tubules andthen metal microcylinders which are electrically conducting and/ormagnetically sensitive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a histogram of lengths of the DC-8,9-PC lipid tubulesprecipitated from 70% ethanol.

[0013]FIG. 2 is a graph of the metal microcylinders after dehydrationand drying, showing length in FIG. 2A and width or outside diameter inFIG. 2B, determined by scanning electron microscopy, wall thickness inFIG. 2C, determined by transmission electron microscopy, and an idealhypothetical metal microcylinder of this invention FIG. D.

DETAILED DESCRIPTION OF THE INVENTION

[0014] This invention pertains to a process for preparing metalmicrocylinders and to the metal microcylinders produced thereby.

[0015] An application for the metal microcylinders of this invention isin a plastic sheet used in antenna isolation for weight reduction andelectrical properties wherein amount of the metal microcylinders in theplastic sheet is below the percolation threshold. Another application isin stealth components which absorb radiation, where amount of the metalmicrocylinders is at about the percolation threshold.

[0016] Adding electrically conductive particles to an insulating polymerincreases the permittivity and conductivity of the resulting composite.When sufficient particles have been loaded, the composite will itselfbegin to conduct electricity over macroscopic distances. The onset ofthis transformation is called percolation.

[0017] Process for making the metal microcylinders of this invention ischaracterized by the use of technical grade diacetylinic lipids which donot require a preliminary purification, and by electroless plating in aplating bath of self-assembled lipid tubules until exhaustion of theplating bath, as evidenced by cessation of bubbling. The metalmicrocylinders are electrically conductive and/or magnetically sensitiveand are typically devoid of the lipid which was used to make the lipidtubules.

[0018] The initial step in the process is dissolution of a diacetyleniclipid in a solvent for the lipid. If heating is resorted to forsolubilization, the temperature must be such as to avoid thermaldegradation of the lipid. Typically, a lower alkyl alcohol, such asethanol, is used to solubilize the lipid at room temperature. In orderto avoid the preliminary purification, the lipid is initially dissolvedin a solvent at a temperature of about 60-70° C., which is above thetransition temperature, and the lipid material that either does notdissolve or that forms a syrup layer is removed, as by filtration and/ordecantation of the dissolved lipid. The dissolved lipid is added tofresh solvent containing water and held at or incubated at an elevatedtemperature above the transition temperature for a period exceeding aquarter of one hour to many hours, or overnight, and then slowly cooledto room temperature over a period of many hours to allow forself-assembly of the lipid into tubules. The presence of water with thesolvent allows for self-assembly of the lipid. During cooling, thedissolved lipid is agitated, as by inversion of its container once ortwice a day. With some lipid batches, a solid precipitate forms at thebottom of the container during the incubation or cooling periods. Thissolid precipitate is also removed, as by filtering the tubules through afilter.

[0019] In a particular case, a volume of 20 liters of 70% ethanol, i.e.,a solution consisting of 70% ethanol and 30% deionized water, on volumebasis, was prepared in a container from 95% ethanol. One liter of 95%ethanol was placed into a separate flask and 100g of technical grade asreceived diacetylenic lipid, i.e., 1,2-bis(10,12tricosadiynoyl)-sn-glycero-3-phosphocoline, was added thereto and heatedto 60-70° C. until the lipid dissolved, which took about 15 minutes.Material that did not dissolve or formed a syrup layer was rejected byfiltering and or decanting the dissolved lipid. The dissolved lipid wasadded to the remainder 19 liters of the 70% ethanol and held at 60° C.overnight. Thereafter, the temperature was reduced to 45° C. and then atone degree per day to room temperature. The slow cooling of thedissolved lipid took 20 days and facilitated self-assembly of the lipidinto tubules. The container, containing the lipid dissolved in the 20liters of the 70% ethanol, was agitated by inverting it once or twice aday. A solid precipitate formed at bottom of the container. Theprecipitate was also rejected or removed by filtering the lipid tubulesthrough a wire mesh.

[0020] After formation by self-assembly, the tubules are collected bycentrifugation or allowed to settle by gravity and the supernatant isdecanted resulting in a tubule dispersion in a water/solvent liquid ofreduced volume. The volume reduction is typically 75-85%. The reducedvolume of the tubule dispersion is dialyzed against water several timesover a period of several days at room temperature in order to replacethe solvent with water. On dialysis, the volume of the tubule dispersiontypically increases due to swelling of the dialysis tubing. Afterdialysis, typically have 50-100 g of lipid tubules in 1-20 liters ofwater. After completing dialysis, tubules are again collected anddispersed in several liters of water. The tubules at this stage are notrobust, i.e., not strong enough to retain their structure upon removalfrom the liquid they are in and collapse upon removal from the liquid.

[0021] In a particular case, the tubules were allowed to settle bygravity and the supernatant was decanted leaving a reduced volume of 3-5liters of tubule dispersion, i.e., the tubules in the 70% ethanol. Thetubule dispersion, which was clear, was dialyzed against water half adozen times over a period of three days at room temperature to replaceethanol with water. The volume of the tubule dispersion about doubleddue to swelling of the dialysis tubing.

[0022] Pre-metallization of the tubules in water involves addition of acrystalline salt to reduce pH of the clear tubule dispersion to the acidside followed by addition of a Pd-Sn catalyst, which is in the form of abrown liquid. The salt functionalizes the tubule surfaces and preparesthem for the catalyst and the catalyst activates charged tubule surfacesfor metal deposition. After addition of the salt and the catalyst, whatresults is a brown suspension which is held for a period of time of onequarter of one hour to overnight, or about 16 hours. During the holdingphase, the tubules absorb the catalyst and become brown while the liquidportion of the suspension becomes clear. At this stage, a series ofwashes with deionized water is carried out to remove excess catalyst andthe salt. The washings are continued until traces of the brown color areremoved and then additional washings are made to remove any remainingtraces of the catalyst and the salt. For purposes of distinction, afterabsorption of the catalyst by the tubules, the tubule dispersion isreferred to herein as the tubule suspension, although a dispersion and asuspension have similar chemical connotations.

[0023] To determine amount of the lipid in the suspension, a smallaliquot of the suspension is removed and dried. This is done in thisfashion since drying destroys the lipid tubule structure.

[0024] In a particular case, the salt was Shipley's Cataprep 404 andamount of the salt added to the tubule dispersion with mixing was 270grams per liter of the dispersion, the dispersion containing 30 grams oflipid tubules per liter of the dispersion . Before addition of the salt,pH of the dispersion was about 7 or neutral and after addition of thesalt, it was about 4. Following this, the Pd-Sn catalyst, i.e.,Shipley's Cataposit 44, was added slowly to a final concentration of0.9% by volume. After addition of the catalyst, the tubules were allowedto settle overnight and became brown by absorption of the catalyst. Thesupernatant was discarded each day and additional water was added untilthe supernatant became clear of detectable brown color and thencontinued for additional three days. The washes were checked forcompleteness empirically by testing the batch in a plating bath. Thegeneration of large amounts of debris in the plating bath is evidence ofincomplete washing. The washed catalyzed tubules were stored in water atroom temperature.

[0025] Electroless plating of the lipid tubules is conducted usingconventional commercial metallization reagents. The plating bath isprepared by adding with mixing to a vessel water, metallization reagentsand the suspension containing lipid tubules which were pretreated withthe salt and the catalyst. Sufficient amounts of the metallizationreagents must be added to obtain a metal coating of sufficient thicknessto make the tubules electrically conducting and robust. The lipidtubules in the plating bath before plating is commenced are brown andthe liquid in the bath corresponds to the color of the metallizationreagents, which is blue in the case of copper metallization. Typically,0.75-1 gram of lipid tubules is used per 10 liters of plating bath. Themetallization reaction commences with spontaneous bubbling and continuesuntil bubbling stops, indicating exhaustion of the plating bath. Duringplating, the tubules undergo a color change that depends on the color ofthe metal plated. Duration of the electroless plating is typically 1-4hours at room temperature. Bubbling commences in about 5 minutes afterall components are placed into the bath.

[0026] Any electrically conducting or ferromagnetic metal or both can bedeposited on the tubules and its thickness should be sufficient torender the tubules electrically conducting and/or magneticallyeffective. Thus, by plating on the tubules an electrically conductingmetal, such as copper, highly electrically conducting tubules can beformed. However, by plating on the tubules a magnetic metal, such asnickel, tubules of low electrical conductivity but of high magnetism canbe obtained. By plating both an electrically conducting metal and amagnetic metal, tubules can be produced with high electricalconductivity and high magnetism. In order to deposit sufficientthickness of the metal, plating is prolonged until bubbling stops,indicating exhaustion of the bath.

[0027] In a particular case, an electroless or chemical copper platingbath was composed of 8 liters of deionized water with 1 liter of each ofShipleys Cuposit 328 A and 328Q metallization reagents. A 10-liter bluebath was used to plate almost 1.0 g of catalyzed tubules. The bath wassubjected to occasional stirring during plating. Reaction in the bathwas commenced with bubbling and proceeded until the bath was exhausted,as evidenced by the loss of the blue color conferred to it by presenceof copper ions and cessation of gas generation or bubbling. Aftercompletion of plating, the tubules were collected by filtration andwashed several times with water to remove the plating liquid.

[0028] If overplating is desired to deposit another coat of metal, itcan be done at this point by collecting the tubules and placing theminto another plating bath containing metallization reagents whichdeposit the desired metal and the plating operation is repeated todeposit a coating of another metal on the initial metal coat.

[0029] Overplating can be used to advantage here. The first coat can beelectrically conductive making the tubules electrically conductivewhereas it may be desired to render the tubules also magneticallysensitive. This can be achieved by plating a coating of another metalwhich renders the tubules magnetically sensitive. In such a case, theresult is tubules which are not only electrically conductive but arealso magnetically sensitive. Another reason for overplating metallizedtubules is for the protection which the initial metal coating confers.After initial plating, it may be desired to overcoat metallized tubulesat a temperature above or below the initial plating carried out at roomtemperature. Overplating may also be resorted to for the reason that asubsequent metal coat is incompatible with the lipid disposed below theinitial metal coat.

[0030] Metallized tubules are then washed with a solvent for lipid, suchas a lower alkyl alcohol, in order to remove lipid and convert thetubules to metal microcylinders which typically have an aspect ratio ofless than the lipid tubules. Removal of the lipid is apparently effectedthrough the breaks in the metal coating on the tubules or through theopen ends of the tubules or some other way and results in a free-flowingproduct after removal of most or all of the sticky lipid. Reduction ofthe aspect ratio of the metal microcylinders as compared to the tubulesresides in the fact that there is considerable breakage of the tubulesduring metallization.

[0031] In a particular case, after metallization, the tubules werecollected by filtration of the plating bath and then washed three timeswith about 2 liters water to remove traces of the plating bath. This wasfollowed by washings with about 2 liters methanol three times todissolve and remove the lipid and then with about 2 liters of acetonefor dehydration purposes. The product was dried under flowing nitrogenand heated in a container in a hot water bath. After drying, the productwas a fine, free-flowing powder of individual robust metalmicrocylinders which was stored in a nitrogen atmosphere at roomtemperature.

[0032] Length of the metal microcylinders produced from the DC-8,9-PCdiacetylenic lipid, as described herein, is shown in FIG. 2A which showsthe length varying from less then 10 to 80μ, with majority of themicrocylinders being 10-40μ, with the weight average length of about20μ. Width or outside diameter is shown in FIG. 2B where width is shownas varying from about 1 to about 3.5μ, with majority of themicrocylinders having width between about 1μ and about 2.5μ, and theweight average width being about 1.5μ. Wall thickness is also avariable, as shown in FIG. 2C, where it is shown as varying from about0.1μ to about 0.5μ, with most of the microcylinders having wallthickness of about 0.2 to about 0.4μ, and the weight average wallthickness being about a quarter of a micron.

[0033]FIG. 2D shows dimension of a hypothetical ideal hollow metallicmicrocylinder 200 having length 202 of 19.4μ, width or outside diameter206 of 1.59μ, and wall thickness 208 of 0.26μ. Based on micrographs ofthe tubules, wall 208 is hollow due to the fact that the lipid whichformed the lipid tubule was removed by washing with a solvent. Thehollow ring in the microcylinder wall, formed by removal of the lipidafter metallization, is a fraction of the wall thickness and isestimated to be on the order of 0.01μ when present.

[0034] It should be understood that depiction of the hypothetical metalmicrocylinder in FIG. 2D is idealized and actual metal microcylindersare far removed from what is shown in FIG. 2D. For instance, actualmetal microcylinder have a length that is far removed from the 19.4μ, asevident from FIG. 2A.

[0035] While presently preferred embodiments have been shown of thenovel transducer preforms and transducer elements, and of the severalmodifications discussed, persons skilled in this art will readilyappreciate that various additional changes and modifications may be madewithout departing from the spirit of the invention as defined anddifferentiated by the following claims.

What is claimed is:
 1. A process for preparing metal microcylinderscomprising the steps of (a) dissolving a diacetylenic lipid in a solventfor the lipid to form a lipid solution, (b) adding water to the lipidsolution to form a lipid solvent/water solution, (c) heating the lipidsolvent/water solution above transition temperature for the lipid andallowing the lipid to self-assemble into lipid tubules to form tubulesolvent/water dispersion, (d) removing solvent from the tubulesolvent/water dispersion to form tubule water dispersion, (e) adding asalt to the tubule water dispersion to reduce pH to acidic, (f) adding aPd-Sn catalyst to the acidic tubule water dispersion to form a tubulewater suspension, (g) adding the tubule water suspension to anelectroless plating bath containing water and at least one metal platingreagent, (h) electroless plating the metal onto the surfaces of thetubules to make metallized electrically conducting tubules, (i)separating the metallized electrically conducting tubules from theplating bath, and (j) removing the lipid from the metallizedelectrically conducting tubules to form metal microcylinders. 2.Theprocess of claim 1 including the step of removing the material that didnot dissolve in the solvent or that formed a syrup layer after the lipidwas dissolved in the solvent.
 3. The process of claim 1 where theelectroless plating step is continued through commencement of bubblingto cessation of bubbling which is indicative of exhaustion of the bath.4. The process of claim 1 wherein the step of removing the lipid in step(j) of claim 1 includes the step of washing the metallized lipid tubulesin a solvent for the lipid.
 5. The process of claim 1 wherein thesolvent for the lipid in step (a) of claim 1 is an alcohol, the heatingin step (c) of claim 1 is conducted to a temperature of 60-70° C. andheld above the transition temperature for sufficient duration forself-assembly of the lipid to take place and form lipid tubules, thestep of removing the solvent from the tubule solvent/water dispersion instep (d) of claim 1 is effected by dialysis with water, the salt in step(e) of claim 1 is pretreatment for the catalyst, amount of Pd-Sncatalyst in step (f) of claim 1 is 90 mls per 10 liters of the tubulesuspension, and the separating step of step (i) of claim 1 is effectedby decantation and/or water washing.
 6. The process of claim 1 includingthe step of removing the material that did not dissolve in the solventor that formed a syrup layer after the lipid was dissolved in thesolvent; wherein the electroless plating step is continued throughcommencement of bubbling to cessation of bubbling which is indicative ofexhaustion of the bath; wherein the step of removing the lipid in step(j) of claim 1 includes the step of washing the metallized lipid tubulesin a solvent for the lipid.
 7. The process of claim 5 including the stepof removing the material that did not dissolve in the solvent or thatformed a syrup layer after the lipid was dissolved in the solvent;wherein the electroless plating step is continued through commencementof bubbling to cessation of bubbling which is indicative of exhaustionof the bath; and wherein the step of removing the lipid in step (j) ofclaim 1 includes the step of washing the metallized lipid tubules in asolvent for the lipid.
 8. The process of claim 6 wherein the lipid isDC-8,9-PC; and the step of removing the lipid from the metallizedelectrically conducting tubules to from the metallic microcylinders iseffected with in methanol.
 9. The process of claim 8 including the stepof washing the metallic microcylinders with acetone after the lipid isremoved with methanol.
 10. The process of claim 1 wherein the tubulewater dispersion contains 50-100 grams lipid tubules per 1-20 liters ofthe dispersion; amount of the salt added in step (e) of claim 1 is about270 grams per liter of the tubule water dispersion; and amount of thePd-Sn catalyst added in step (f) of claim 1 is about 90 mls per 10liters of the acidic tubule water dispersion.
 11. The process of claim10 including the step of holding the tubule water dispersion at roomtemperature for a period of ¼-16 hours for the tubules to absorb thecatalyst and become brown while color of the dispersion changes frombrown to clear and washing the brown tubules with water to remove thecatalyst and/or the salt.
 12. The process of claim 1 including the stepof overplating another metal on the metallized electrically conductingtubules following step (i) of claim
 1. 13. A process of electrolessplating of diacetylenic lipid tubules which are brown and have beenpretreated with a salt and a Pd-Sn catalyst to from electricallyconducting metal micocylinders, the process comprising the steps of (a)placing the tubules into a plating bath containing at least one metalplating reagent, (b) commencing metal plating as indicated by gasgeneration and bubbling in the plating bath, (c) terminating the metalplating as evidenced by cessation of gas generation and bubbling in theplating bath, (d) separating the metallized electrically conductingtubules from the plating bath, and (e) removing the lipid from themetallized tubules to form the electrically conducting metalmicrocylinders.
 14. The process of claim 13 wherein amount of thetubules is 0.75-1 gram per 10 liters of the plating bath, amount of theat least one metal plating reagent is such as to deposit sufficientmetal on the tubules by means of the electroless plating to make themelectrically conducting.
 15. The process of claim 14 wherein theseparating step of step (d) of claim 13 is effected with water washes toremove the plating bath.
 16. The process of claim 15 including the stepof overplating another metal on the metallized electrically conductingtubules following step (d) of claim
 13. 17. The process of claim 16wherein electroless plating of a metal on the pre-treated lipid tubulesto form metallized electrically conducting tubules takes 1-4 hours atroom temperature.
 18. Hollow electrically conducting and/or magneticmetal microcylinders comprising length of from less than 10 to 80μ,outside diameter of from about 1 to 3.5μ, and wall thickness of fromabout 0.1 to 0.5μ.
 19. The microcylinders of claim 18 having aspectratio of about 12 wherein weight average length is about 1.5μ and weightaverage wall thickness is about a quarter of a micron and it is hollow.20. The microcylinders of claim 18 having a layer of another metal overthe metallized surfaces of said microcylinder.